99 Tahoe problems

Hi, I have a 99 chevy tahoe that has been running very poorly for some months now. I first noticed I had problems when there seemed to be a very bad misfire and the fuel consumption was worsening. I am currently putting an average of $500 a month into the tank! I was told to have an oil change and the plugs and wires done. I've had those things done and the problems are still there as well as it using even more gas it also seems to have a slight backfire that was not present before the work. The vehicle idles very rough while in park and while stopped in drive. The gas guage also is not reading so I am guessing to how much fuel I have. I was told that one of my spark plugs had a "non fouler" extension and that the extension and plug were covered in oil. The mechanic did not put another "non fouler" on when he changed the plugs. The rest of the plugs looked normal but still were replaced. There definitely is some power loss and it gets very close to possibly stalling out, the lights on dash seem to flicker or surge when this happens. I would appreciate any help on this because at this point I don't know what's going on. In the past year and half I have replaced the fuel pump, water pump, radiator, thermostat, intake gasket, pair of oil lines, some sort of seal that's located behind water pump, control unit for the heat/air and that's just off the top of my head.